Shear walls play an important role in residential and commercial construction. If one considers a simple box structure having panels fastened to framing, it can be seen that a strong lateral force acting against one side of the box (e.g., wind pressure) will tend to force the side walls resisting that force from a rectangular shape into a parallelogram. Not all sheathing panels are capable of resisting such forces, nor are they very resilient, and some will fail, particularly at points where the panel is fastened to the framing. Where it is necessary to demonstrate shear resistance, the sheathing panels are measured to determine the load that the panel can resist within the allowed deflection without failure.
The shear rating is generally based on testing of three identical 8×8 ft (2.44×2.44 m) assemblies, i.e., panels fastened to framing. One edge is fixed in place while a lateral force is applied to a free end of the assembly until the load is no longer carried and the assembly fails. The measured shear strength will vary, depending upon the thickness of the panel and the size and spacing of the fasteners used in the assembly. For example, a typical assembly, e.g., a nominal ½ inch (12.7 mm) thick plywood fastened with 8d nails (see the nail description below) to nominal 2×4 inch (50.8×101.6 mm) wood studs spaced 16 inches (406.4 mm) apart (on centers), the nails being spaced 6 inches (152.4 mm) apart on the perimeter and 12 inches (304.8 mm) apart within the perimeter, would be expected to show a shear strength of 720 lbs/ft (1072 kg/m) before failure occurs. (Note that the measured strength will vary as the fastener size and spacing is changed, as the ASTM E72 test provides.) This ultimate strength will be reduced by a safety factor, e.g., typically a factor of three, to set the design shear strength for the panel.
Sheathing panels used where a shear rating must be met usually are plywood or oriented strand board (OSB), which consists of pieces of wood that are glued together. These panels can provide the needed shear strength, but each is combustible and neither is durable when exposed to water. A panel made of hydraulic cement will resist water, but is much heavier than the wood panels and has insufficient shear strength. To solve some of these problems, structural cement panels (SCP's or SCP panels) were developed.
U.S. Pat. No. 6,620,487 to Tonyan et al., incorporated herein by reference in its entirety, discloses a reinforced, lightweight, dimensionally stable structural cement panel (SCP's or SCP panels) capable of resisting shear loads when fastened to framing equal to or exceeding shear loads provided by plywood or oriented strand board panels. The panels employ a core of a continuous phase resulting from the curing of an aqueous mixture of calcium sulfate alpha hemihydrate, hydraulic cement, an active pozzolan and lime, the continuous phase being reinforced with alkali-resistant glass fibers and containing ceramic microspheres, or a blend of ceramic and polymer microspheres, or being formed from an aqueous mixture having a weight ratio of water-to-reactive powder of 0.6/1 to 0.7/1 or a combination thereof. At least one outer surface of the panels may include a cured continuous phase reinforced with glass fibers and containing sufficient polymer spheres to improve nail ability or made with a water-to-reactive powders ratio to provide an effect similar to polymer spheres, or a combination thereof.
U.S. Pat. No. 6,241,815 to Bonen, incorporated herein by reference in its entirety, also discloses formulations useful for SCP panels.
U.S. patent application Ser. No. 10/666,294, incorporated herein by reference, discloses a multi-layer process for producing structural cementitious panels (SCP's or SCP panels), and SCP's produced by such a process. After one of an initial deposition of loosely distributed, chopped fibers or a layer of slurry upon a moving web, fibers are deposited upon the slurry layer. An embedment device mixes the recently deposited fibers into the slurry, after which additional layers of slurry, then chopped fibers are added, followed by more embedment. The process is repeated for each layer of the board, as desired.
However, while using the SCP panel on a wooden frame is an improvement over using plywood it would be desirable to have a further noncombustible system.
For use in construction, SCP panels should meet building code standards for shear resistance, load capacity, water-induced expansion and resistance to combustion, as measured by recognized tests, such as ASTM E72, ASTM E661, and ASTM C 1185 or equivalent, as applied to structural plywood sheets. SCP panels are also tested under ASTM E-136 for non-combustibility—plywood does not meet this test.
The SCP panel should be capable of being cut with the circular saws used to cut wood.
The SCP panel should be dimensionally stable when exposed to water, i.e., it should expand as little as possible, preferably less than 0.1% as measured by ASTM C 1185.
The SCP panel should provide a bondable substrate for exterior finish systems.
The SCP panel should be non-combustible as determined by ASTM E136.
After curing for 28 days, the flexural strength of a 0.75 inch (19 mm) thick SCP panel having a dry density of 65 lb/ft3 (1041 kg/m3) to 90 lb/ft3 (1442 kg/m3) or 65 lb/ft3 (1041 kg/m3) to 95 lb/ft3 (1522 kg/m3) after being soaked in water for 48 hours should be at least 1000 psi (7 MPa), e.g. at least 1300 psi (9 MPa) preferably at least 1650 psi (11.4 MPa), more preferably at least 1700 psi (11.7 MPa), as measured by ASTM C 947. The panel should retain at least 75% of its dry strength.
As the thickness of the board affects its physical and mechanical properties, e.g., weight, load carrying capacity, racking strength and the like, the desired properties vary according to the thickness of the board. Thus, the desired properties which a shear rated panel with a nominal thickness of 0.5 inches (12.7 mm) should meet include the following.
The panel when tested according to ASTM E661 and American Plywood Association (APA) Test Method S-1 over a span of 16 inches (406.4 mm) on centers, should have an ultimate load capacity greater than 550 lbs (250 kg) under static loading, an ultimate load capacity greater than 400 lbs (182 kg) under impact loading and a deflection of less than 0.078 inches (1.98 mm) under both static and impact loading with a 200 lb (90.9 kg) load.                The nominal racking shear strength of a 0.5 inch (12.7 mm) thick panel measured by the ASTM E72 test using the nail size and spacing described above should be at least 200 lbs/ft (about 300 kg/m), typically at least 720 lbs/ft (1072 kg/m).        A 4×8 ft, ½ inch thick panel (1.22×2.44 m, 12.7 mm thick) should weigh no more than 99 lbs (44.9 kg) or 104 lbs (47 kg) and preferably no more than about 96 or 85 lbs (about 44 or 39 kg).        The panel should be capable of being cut with the circular saws used to cut wood.        The panel should be capable of being fastened to framing with nails or screws.        The panel should be machinable so that tongue and groove edges can be produced in the panel.        The panel should be dimensionally stable when exposed to water, i.e., it should expand as little as possible, preferably less than 0.1% as measured by ASTM C 1185.        The panel should not be biodegradable or subject to attack by insects or rot.        The panel should provide a bondable substrate for exterior finish systems.        The panel should be non-combustible as determined by ASTM E136.        After curing for 28 days, the flexural strength of a 0.5 inch (12.7 mm) thick panel having a dry density of no more than 65 to 95 lb/ft3 (1041 to 1520 kg/m3) after being soaked in water for 48 hours should be at least 1700 psi (11.7 MPa), preferably at least 2500 psi (17.2 MPa), as measured by ASTM C 947. The panel should retain at least 75% of its dry strength.        
It should be evident that plywood and OSB panels meet some, but not all, of the above performance characteristics.
There is a need for an economical, easy to assemble, durable and non-combustible total framing and shear wall system.